When I worked DSCS-III satellite operations, we used ground stations around the world to control the geosynch birds. The antennas were either 32, 40, or 60 feet in diamater (depending on the station) and used a 1 KW uplink. In addition, we had to have the proper command protocols to do anything. You couldn't just send up any set of bits and get it accepted by the command decoder.

Mars is over 1000 times as far away as geosynch. While I'm not certain, I doubt if the proper command protocols for MER are available to anyone in the public. Even if they were, you'd need a big antenna and a lot of transmitter power to reach Mars with enough signal strength to do anything. There really aren't very many antennas with the right combination of power and frequency on Earth to hack MER. It's a really, really unlikely scenario.